As is known in the art, a nerve block is achieved by delivering a local anesthetic solution proximate to a nerve to be blocked. Typically, the anesthetic solution is delivered by inserting a needle such that the anesthetic solution can be injected in close proximity to the nerve to be blocked. To correctly position the needle, it is necessary to identify the nerve location prior to the injection of the local anesthetic solution.
One conventional technique used to correctly position the needle proximate to a nerve is to elicit paresthesic. This technique, however, is not a reliable way to position the needle proximate to the nerve because of individual traits of each patient, such as patient nervousness, patient sedation, patient communication skills and/or cooperation. In addition, some patients may not sense paresthesia, and thus, if the needle is inserted into a motor nerve bundle, nerve damage may occur due to the tip of the needle being too close to the motor nerve or motor nerve bundle or the needle being inserted directly into the nerve, while injecting the local anesthetic solution. The elicitation of paresthesia technique, as described above, is also very time consuming.
In attempts to provide alternatives to the above-described elicitation of paresthesia technique, mechanical aids including radioscopy and a peripheral nerve stimulation (PNS) technique have been introduced into practice. The radioscopy technique includes using the X-ray to find the bones as landmarks and to position the needle under X-ray assuming the operator knows the spatial relation between the bone landmark and the nerve to be blocked. There are several drawbacks for this technique including: it requires X-ray equipment; the patient has to be exposed to X-ray; the operator needs to be trained to use X-ray equipment; the end of the point of approximation of the needle to the nerve is not defined prior to delivery of local anesthetics; and it is also time consuming.
The PNS technique uses the basic principle that a rectangular electrical pulse having a predetermined threshold of direct current and duration can depolarize a nerve cell membrane. In practice, the PNS technique uses an insulated needle as a negative electrode, which is inserted into human tissue in a region to receive a nerve blocking solution. The PNS technique further uses a positive electrode, which is placed somewhere on the patients skin as a current return path. An electrical pulse is emitted from the tip of the needle and travels through the tissue to the positive electrode. If the nerve to be blocked is located in between these two electrodes and the current density (current per unit of cross-section area) and the pulse width are at or above the predetermined threshold, the electrical pulse causes depolarization of the nerve axon, which causes contraction (or a twitch) of the muscle being innervated by the motor nerve. The current required to depolarize the nerve and trigger the corresponding muscle contraction is related to the distance between the tip of the needle and the nerve. This information is used as a guide for positioning the tip of the needle to a location proximate to the nerve to be blocked prior to delivering the local anesthetic. The lower the current required, the closer the tip of the needle to the nerve.
One drawback of the PNS technique is that it uses only one positive electrode, which restricts an operator (e.g., clinician) to obtaining only two-dimensional position information for positioning the tip of the needle close to the nerve. This drawback is further exacerbated in that it is not clear where the positive electrode should be located on the patient's skin for providing the electrical current return path for the pulse emitted from the tip of the needle.
Therefore, an unsolved need remains for a system for providing nerve mapping that overcomes the above-described limitations and drawbacks of the prior art.